Quantum Mechanics

[exchemist]

First, I want to thank everyone participating in this excellent thread for such an interesting discussion, and especially @Polymath257 for starting the thread.

Second, you're all such morons for failing to grasp even on the most rudimentary level, as I myself have, the simple fact that quantum mechanics proves beyond doubt aliens are visiting us in order to conduct anal probes on fans of Country and Western music!!!!!

Actually, I don't believe that even for a nanosecond. You pretty much all seem knowledgeable to me. But this is RF, and you were having a civil discussion, so I was alarmed that something might be wrong -- terribly wrong -- here. Hence, my obligatory ignorant and arrogant outburst.

Next, and more seriously, I must honestly apologize for my own ignorance of quantum mechanics, and for the fact I might be asking some dumb questions now or later on -- if and when I'm not merely lurking.

You see, the fact is, QM never interested me until quite recently in life, except for my early on interest in QM as an obvious and exciting means of picking up babes in bars (e.g. "So...is it 'Lisa'?...so Lisa, shall we go to my apartment, have a little more wine, and then discuss 'entanglement' in the privacy of my bedroom? Or would you prefer to entangle on my kitchen table where you'll find even more cheese than in my pick-up lines?"). Hence, I have only recently gotten interested in it, and that mainly for its philosophical implications.

Having said all that, please allow me now to ask two questions.

1) Does the fact (as indeed I understand it to be a fact) that in QM some effects are completely uncaused (e.g. radioactive decay) imply that the universe itself might have been a completely uncaused effect?

2) And, more broadly, if there can be uncaused effects, does that decisively sink once and forever the ancient notion that, "Something cannot come from nothing"?

Thanks for your patience.

Yes a good thread - until the, ahem, less serious contributors arrive, at least.

I suppose one always has to play safe and say that "according to observation", radioactive decay is uncaused. So far as I know, all that means is that "it just happens", i.e. seemingly randomly and that we have no model to account for decay in terms of any influence. But if someone more knowledgeable can say there are theoretical reasons to expect it to be uncaused, i.e. truly random, then I'll be glad to learn.

I don't think myself that radioactive decay being uncaused tells us anything about the origin of the universe, save for what you imply in (2), viz. radioactive decay sets at least a precedent for apparently uncaused phenomena.

 

[shunyadragon]

Depends how long you've been at it. Some of the things I read twice or more when first encountering them were uncaused causes like the Casimir effect, faster-than-light information transfer in entanglement, the reasoning and implications of Bell's theorem; they're weird. (And I wasn't the one who coined 'spooky action at a distance'.)

Only afterwards did I get used to them. Well, as used to them as I am now, anyway.

Uncaused effects are simply the way of Quantum Mechanics, and the likely cause is simply the nature of Quantum Mechanics and the Quantum World itself.

 

[Polymath257]

1) Does the fact (as indeed I understand it to be a fact) that in QM some effects are completely uncaused (e.g. radioactive decay) imply that the universe itself might have been a completely uncaused effect?

2) And, more broadly, if there can be uncaused effects, does that decisively sink once and forever the ancient notion that, "Something cannot come from nothing"?

Thanks for your patience.

Maybe those aliens have the answers....ask them next time they do a probe.....

More seriously, yes. this is partly where the 'universe from nothing' models start. The question is how to get a quantum fluctuation to grow rather than stay small and how spacetime geometry is produced out of this. And yes, the universe *might* have been uncaused.

Are these issues settled? No way. There is still plenty of debate and speculation and not nearly enough data to support the ideas we are playing with.

 

[sealchan]

So.. I skipped the entire classical physics part and learnt and used QM directly. I think of macro reality as averaged out appx. based on statistical mechanics. Thus I don't find QM weird.

Explain why two similarly shaped and sized objects dropped from the same height fall at the same speed...using only quantum mechanics.

Modern physics tells you about what is going on backstage but classical physics tells you about the play itself.

 

[sealchan]

Probability isn’t out of ignorance. The simple example is the double slit experiment. It isn’t a matter of determining which slit it went through, for all intents and purposes it goes through both. The universe is fundamentally not deterministic which effects everything.

At that level...at a higher (?more gravitationally consequential) level things are strongly deterministic. To think otherwise is to abandon common sense and practical thinking.

 

[Bob the Unbeliever]

I interpret the macroscopic causality as being an averaging process over the probabilities from the lower levels. A trivial example is to imagine a coin. Flip it one time and you have a 50/50 chance of a head or a tail, with little predictability (well...this is a classical system, so...). But, if you flip a trillion coins, the number of heads will be very close to 50%. Yes, there is a spread, but as a percentage that spread is small.

If you have Avagadro's number of coins to flip, that 50% is accurate to within our ability to measure. And for macroscopic measurements, we are always dealing with an average over many, many quantum particles. So the predictability is very high. And that is why classical causality works for macroscopic objects.

yes, good contribution!

You explain this so very well-- and this is how I understand macro events, the average of trillions of individual probability events. With that many, the measurements are rock solid.

.................

Back to the coin-flip: what are the odds of an edge-landing, and can we figure that into our final 50% measurement? Or are those odds so small, that they vanish in the trillions of flips?

Or do we presume mechanically flipping the coin directly over a sharp edge, or wire, such that it cannot possibly land on it's edge?

 

[Bob the Unbeliever]

First, I want to thank everyone participating in this excellent thread for such an interesting discussion, and especially @Polymath257 for starting the thread.

Second, you're all such morons for failing to grasp even on the most rudimentary level, as I myself have, the simple fact that quantum mechanics proves beyond doubt aliens are visiting us in order to conduct anal probes on fans of Country and Western music!!!!!

Actually, I don't believe that even for a nanosecond. You pretty much all seem knowledgeable to me. But this is RF, and you were having a civil discussion, so I was alarmed that something might be wrong -- terribly wrong -- here. Hence, my obligatory ignorant and arrogant outburst.

Next, and more seriously, I must honestly apologize for my own ignorance of quantum mechanics, and for the fact I might be asking some dumb questions now or later on -- if and when I'm not merely lurking.

You see, the fact is, QM never interested me until quite recently in life, except for my early on interest in QM as an obvious and exciting means of picking up babes in bars (e.g. "So...is it 'Lisa'?...so Lisa, shall we go to my apartment, have a little more wine, and then discuss 'entanglement' in the privacy of my bedroom? Or would you prefer to entangle on my kitchen table where you'll find even more cheese than in my pick-up lines?"). Hence, I have only recently gotten interested in it, and that mainly for its philosophical implications.

Having said all that, please allow me now to ask two questions.

1) Does the fact (as indeed I understand it to be a fact) that in QM some effects are completely uncaused (e.g. radioactive decay) imply that the universe itself might have been a completely uncaused effect?

2) And, more broadly, if there can be uncaused effects, does that decisively sink once and forever the ancient notion that, "Something cannot come from nothing"?

Thanks for your patience.

I, for one was torn: Mark your post as funny, because it literally made me laugh out loud? Informative, because it was? Friendly? A simple thumbs-up?

I went with funny, mainly because my co-workers give me funny looks when I laugh out loud at the computer screen...

As for your question #2-- "... does that decisively sink once and forever ... "

I will politely remind you that Science Marches Forward, always. Not that long ago? Everyone agreed that Newtonian Physics was the Final Answer to Life's Questions about the Greater Universe.

And then? It wasn't.

So along comes Professor Einstein, and General Relativity. Was Newton therefore ... out? And Relativity In?

Well, no.... not really. Engineers still navigate to Mars, using Newtonian physics, because it works quite well enough in the short term.

It's only when we are looking at long time-frames, or with high relative speeds, or into very dense gravity wells, that Newton fails the test of Predictability. And we need to invoke Einstein's maths.

But Relativity does not seem to work at the sub-atomic levels.... so was Einstein ... wrong? And QM is right?

Hmmmmmmm......

Newton's stuff works-- under certain limited conditions. So, it's not wrong, exactly. But it's not right, either.

Same for Einstein. And likely, the same for QM.

I fully expect that in some future? There will be a newer model/theory, that better describes certain very extreme aspects of our Universe, that QM does not describe accurately....


Bottom line: It all comes down to measurements, doesn't it? The better humans get at measuring the Universe? The better we understand it, and that causes us to need to refine our Models.

Did we discard all of our kitchen knives, when the gun was invented? Well, no-- they are still useful.

Newton is still useful, isn't it? But I doubt that Science will ever get to the status of "once and forever".

 

[sealchan]

Yes a good thread - until the, ahem, less serious contributors arrive, at least.

I suppose one always has to play safe and say that "according to observation", radioactive decay is uncaused. So far as I know, all that means is that "it just happens", i.e. seemingly randomly and that we have no model to account for decay in terms of any influence. But if someone more knowledgeable can say there are theoretical reasons to expect it to be uncaused, i.e. truly random, then I'll be glad to learn.

I don't think myself that radioactive decay being uncaused tells us anything about the origin of the universe, save for what you imply in (2), viz. radioactive decay sets at least a precedent for apparently uncaused phenomena.

The weak force is understood to be the mechanism.

This suggests to me a principle of the design of the universe that has few, if any, exceptions...all things that are made can be unmade...all events/states that can occur have limits to their occurrence. I think that modern physics shows us very much its self-referential design in which first cause is increasingly irrelevant to the older, more relatable clockwork model.

 

[Ponder This]

The waves of the ocean don't 'exist' until you notice them.
How many see the wave upon a wave upon a wave?
Eventually, Shiva's Dance will free us from the snare of illusion.
No thing is ever sitting still.

 

[exchemist]

The weak force is understood to be the mechanism.

This suggests to me a principle of the design of the universe that has few, if any, exceptions...all things that are made can be unmade...all events/states that can occur have limits to their occurrence. I think that modern physics shows us very much its self-referential design in which first cause is increasingly irrelevant to the older, more relatable clockwork model.

Indeed, but I thought that simply gave us the rules governing what reactants form what products, rather than a mechanism in the sense I meant, of an influence accounting for what causes a given decay event to take place. Or does it do that too?

 

[David T]

What I meant was in the spirit of what Richard Feynman says...



In my studies of human cognition I am fairly convinced that concepts we hold in the mind are built up from metaphors from our experience of having a body in a (classical) three dimensional world moving through time. As such quantum mechanics presents us with "truths" which are either hard to or impossible to understand via such embodied concepts.

In short, quantum mechanics is weird...even if it is concisely described by mathematics.

I stick to Feynman it works, the rest is just speculation.

 

[sealchan]

Indeed, but I thought that simply gave us the rules governing what reactants form what products, rather than a mechanism in the sense I meant, of an influence accounting for what causes a given decay event to take place. Or does it do that too?

I call it the mechanism because it explains what happens...but as with anything else at the subatomic level at that level there isn't any way of knowing to say "look out that atom is about to crack!"

 

[tayla]

One question is how our classical ideas of causality have to be modified in light of the discoveries of quantum theory. In particular, the fact that initial conditions do NOT determine later states means that the classical concept of causality needs at least some massage.

Seems to me, if the position of the electron (for example) after wave function collapse is non-deterministic (random), there is no such thing as strict causality.

 

[exchemist]

I call it the mechanism because it explains what happens...but as with anything else at the subatomic level at that level there isn't any way of knowing to say "look out that atom is about to crack!"

Right, so each event is random, i.e. uncaused.

This interests me because with atomic emission spectra there are two types of emission: stimulated and spontaneous. Stimulated emission is induced by - I would say "caused by" - the effect of a passing photon. Spontaneous emission is not - it just "happens"....or that is what we learnt at university. But actually, I understand it is now attributed to stimulation too, but by random vacuum fluctuations rather than actual photons passing by. So "spontaneous emission" it is not exactly "uncaused", but it is still random, because the vacuum fluctuations themselves are "uncaused".

Is there an analogous mechanism for "triggering" radioactive decay processes, do you know?

 

[Polymath257]

Explain why two similarly shaped and sized objects dropped from the same height fall at the same speed...using only quantum mechanics.

Modern physics tells you about what is going on backstage but classical physics tells you about the play itself.

Only quantum mechanics? Well, there is no quantum theory of gravity, so we use the classical approximation for the potential. And, for anything significantly above the Planck scale (in other words, anything we can test), this is good enough. Alternatively, we do QM on a GR background. Again, as long as the gravity from the quantum fluctuations isn't significant (which, again, is for anything above the Planck scale), this is a perfectly good description.

 

[Polymath257]

Right, so each event is random, i.e. uncaused.

This interests me because with atomic emission spectra there are two types of emission: stimulated and spontaneous. Stimulated emission is induced by - I would say "caused by" - the effect of a passing photon. Spontaneous emission is not - it just "happens"....or that is what we learnt at university. But actually, I understand it is now attributed to stimulation too, but by random vacuum fluctuations rather than actual photons passing by. So "spontaneous emission" it is not exactly "uncaused", but it is still random, because the vacuum fluctuations themselves are "uncaused".

Is there an analogous mechanism for "triggering" radioactive decay processes, do you know?

Well, the spontaneous emission can be thought of that way because any real particle (an electron) is always surrounded by a 'cloud' of virtual particles (for electrons, mostly photons, but a a few virtual electron-positron pairs). This is a result of the EM force.

For alpha decays, the reaction is better described as a tunneling of a helium nucleus out of a nuclear potential well. The same is true for fission reactions. Alternatively, the liquid drop model describes when the nuclear 'drop' will break apart. This is primarily a result of the strong force.

Gamma decay is just another type of photon emission, although for nuclear energies rather than atomic energies. So it has the same types of descriptions as atomic emissions.

Beta decay is mediated by the weak nuclear force. For a standard beta decay, a down quark changes into an up quark and a negative W particle (one of the mediators of the weak force). Then the W particle decays into an electron and a anti-neutrino. The end result is that a neutron (two downs and an up quark) changes into a proton (two ups and a down), an electron, and an anti-neutrino.

This beta decay isn't as easy to describe in terms of an interaction with quantum fluctuations and really is an 'uncaused' decay.

In fact, it is frequently the weak nuclear force that is the 'odd one out' at this level. It is the one that breaks time reversal symmetry. It is the one that break symmetry between left and right handed descriptions. And it is the one that acts different for particles and anti-particles. There are many mysteries here.

 

[sealchan]

Right, so each event is random, i.e. uncaused.

This interests me because with atomic emission spectra there are two types of emission: stimulated and spontaneous. Stimulated emission is induced by - I would say "caused by" - the effect of a passing photon. Spontaneous emission is not - it just "happens"....or that is what we learnt at university. But actually, I understand it is now attributed to stimulation too, but by random vacuum fluctuations rather than actual photons passing by. So "spontaneous emission" it is not exactly "uncaused", but it is still random, because the vacuum fluctuations themselves are "uncaused".

Is there an analogous mechanism for "triggering" radioactive decay processes, do you know?

No idea on that...but fascinating to think that the spontaneous emissions "reflect" the vacuum fluctuations...sounds like something that could be turned into a device to measure that "edge of the Universe".

 

[exchemist]

No idea on that...but fascinating to think that the spontaneous emissions "reflect" the vacuum fluctuations...sounds like something that could be turned into a device to measure that "edge of the Universe".

Yes I thought it was quite neat. I see Polymath is saying that while gamma emission may be induced in a similar way to what I have been describing, alpha decay is another kind of randomness, due to the probability of the alpha particle tunnelling out of the potential well that keeps it inside the nucleus.

Both processes are thus attributable to purely QM concepts and are intrinsically random, i.e. each individual event is "uncaused", although as you rightly point out we do have a "cause" in terms of the QM mechanism behind the process.

This is all rather good stuff, I must say.

 

[sayak83]

Explain why two similarly shaped and sized objects dropped from the same height fall at the same speed...using only quantum mechanics.

Modern physics tells you about what is going on backstage but classical physics tells you about the play itself.

We have to use GR to get to the correct explanation here though.

 

[james blunt]

We have to use GR to get to the correct explanation here though.

No, field density.